What the World’s Largest Organism Reveals About Fires and Forests

This story originally appeared on Atlas Obscura and is part of the Climate Desk collaboration.Under the Blue Mountains of Oregon lurks something massive and prehistoric. Yet the largest recorded organism on Earth, weighing more than 200 blue whales and dwarfing even Pando, Utah’s famous grove of quaking aspens, is nearly invisible to the untrained eye. It’s a single, genetically identifiable specimen of honey mushroom, or Armillaria ostoyae, that has been growing for thousands of years.Nicknamed the Humongous Fungus, it covers nearly 4 square miles within Malheur National Forest and weighs perhaps 7,500 tons (some estimates range as high as 35,000 tons). The fungus likely attained its record-setting dimensions in part thanks to conditions created by 20th century forest management. And it continues to grow, expanding mostly underground in networks of thin filaments called mycelia. As the fungus spreads, it moves up into trees, hidden beneath their bark. It then slowly eats away at its host, often killing the tree and then continuing to munch on the dead wood for decades. More than just an insidious parasite, the Humongous Fungus is a symbol of an ailing, at-risk forest, unintended consequences of fire suppression, and the challenge of restoring an ecosystem’s health.“If there were no trees dying, I wouldn’t have a job,” says forest pathologist Mike McWilliams, who calls himself the unofficial tour guide of the massive fungus. “But I like this thing because it’s super interesting.”McWilliams, whose official duties center around conservation efforts at Malheur, meets visiting researchers (and the occasional curiosity seeker) along US Highway 26, where a country store under towering pines advertises its famous huckleberry ice cream and buffalo burgers. From there, he leads the way along one Forest Service gravel road and then another. Eventually, the party must get out to hike.Soon, dense forest gives way to a balding hillside. The few trees here are more spread out, and some are clearly dying—the work not of the Humongous Fungus but rather a smaller relative. In Malheur, there are several Armillaria specimens, and it’s hard to tell with boots on the ground where one fungus ends and another begins. So researchers collect samples and map them genetically.McWilliams continues driving, following dirt roads deeper into the forest, where the trees become smaller and closer together. The ground is littered with fallen trees and brush, what foresters call surface fuel. Then, at last, the tour arrives at the main attraction: the Humongous Fungus.It’s easier to see the decay that Malheur’s most famous resident leaves behind than the fungus itself. What should be a thick and thriving forest is instead a collection of toppled trees, with many more dying. McWilliams uses his Pulaski, an ax-like forestry tool, to chip away at bark and reveal subtle, cream-colored fans on the exposed wood: evidence of the fungus spreading within an infected fir.

“Part of the reason it got so big is because of the history of fire suppression,” McWilliams says, referring to the dominant tenet of the last century of forest management. “Fires would have reduced the proportion of highly susceptible hosts, and you’d have a functional, healthy forest there.”